Christopher W. Meyer

ITS-90 non-uniqueness from PRT subrange inconsistencies over the range 24.56 K to 273.16 K

We have performed calculations to study ITS-90 non-uniqueness from subrange inconsistencies over the range 24.5561 K to 273.16 K, where the scale is defined by an interpolating platinum resistance thermometer (PRT) that is calibrated via sets of defined fixed points. For this work, subrange inconsistency calculations have been performed on eighteen PRTs; fourteen are standard PRTs and four are miniature PRTs. The inconsistency uncertainties, which result from propagation of fixed-point uncertainties, have also been calculated. The calculations show that PRT subrange inconsistencies in the temperature region studied can be as large as 1 mK. We have also studied possible correlations between PRT subrange inconsistencies and other PRT properties/parameters that are simpler to determine; these studies show that there is a correlation between the average magnitude of the inconsistencies and the value of a certain calibration coefficient. Finally, for the range studied we have used a statistical analysis on the inconsistencies of the PRT ensemble to calculate a standard uncertainty to the ITS-90 temperature T90 due to the inconsistencies. Over the temperature intervals 25 K ≤ T90 ≤ 50 K and 100 K ≤ T90 ≤ 200 K, this uncertainty dominates those propagated from fixed-point uncertainties.

Determination of the uncertainties for ITS-90 realization by SPRTs between fixed points

Calibrated standard platinum resistance thermometers (SPRTs) are used to realize the International Temperature Scale of 1990 (ITS-90) from 13.8033 K to 1234.93 K. The SPRTs are calibrated at a series of fixed points, each assigned a temperature on the ITS90, by measuring the ratios of the SPRT resistances at those temperatures to that at the triple point of water (TPW). For realizing the scale with a calibrated SPRT, a user measures the resistance ratio at the unknown temperature and uses ITS-90-defined equations to interpolate between fixed points. The uncertainty of the SPRT temperature is therefore largely influenced by the propagation of fixed-point resistance-ratio uncertainties. In this paper, we rigorously derive the equations for calculating these uncertainties for a variety of circumstances and we use software tools written by us to perform these calculations using realistic uncertainties for fixed points and other input parameters. For properly calculating the standard uncertainty for SPRT realization of the ITS-90, correlations between the input quantities must be considered, in particular those involving measurement of the TPW resistance. The proper calculation depends on three factors involving SPRT use and calibration. The different combinations of these factors result in six different equations for calculating the realization uncertainty. We derive these six equations, specify the conditions of their use and discuss the relevant uncertainty components for each of them. We also compare the results of these equations with those of two approximations that may be used for calculating the standard uncertainty and explain the conditions under which the simpler approximations agree with the more detailed calculations. Because these calculations are complicated, we are making our software tools available upon request to the user community.

The second-generation NIST standard hygrometer

A second-generation standard hygrometer has been completed at the National Institute of Standards and Technology (NIST). This hygrometer measures humidity using a gravimetric method: it separates the water from the carrier gas and afterwards measures the water mass and carrier gas mass. These two measurements determine the mass ratio r (the ratio of the measured water mass to the measured dry-gas mass). The new design allows automated continuous gas collection at up to 3?L?min?1. This enables the hygrometer to collect larger amounts of gas and thereby measure humidity values lower than that measured by the previous NIST standard hygrometer. When operated in an optimal thermal environment (minimal thermal loads in the laboratory), the total expanded relative uncertainty (k = 2) of the gravimetric hygrometer is approximately 0.1% for atmospheric-pressure frost points higher than ?35??C (r = 250??g?g?1). Below this frost point the total expanded relative uncertainty gradually increases to approximately 1% at ?55??C (r = 13??g?g?1). The hygrometer has measured the humidity of gas samples produced by the NIST Hybrid Generator and the NIST Low Frost-Point Generator with dew/frost points from ?35??C to 71??C. For both generators the differences between the humidity generated and the humidity measured by the gravimetric hygrometer are less than the combined uncertainties of the generator and the hygrometer.

Realization of the ITS-90 at the NIST in the range 0,65 K to 5,0 K using the 3He and 4He vapour-pressure thermometry

The ITS-90 is defined in the ranges 0,65 K to 3,2 K and 1,25 K to 5,0 K by the vapour-pressure/ temperature relations of 3He and 4He, respectively. An apparatus has been constructed at the National Institute of Standards and Technology (NIST) for realizing the ITS-90 below 84 K which includes cells for realizing the ITS-90 through vapour-pressure thermometry with 3He and 4He. We present here the results of our realizations with He vapour pressures. The expanded uncertainties (k = 2) of the ITS-90 temperature realizations are 0,12 mK or less over 97% of the ranges of the ITS-90 definitions. In the lower 3% of each range, the uncertainties of the realizations increase to 0,14 mK for 3He and 0,16 mK for 4He because of the growing size of the thermomolecular pressure correction. Comparisons are made with the previous wire scale of the NIST, which is traceable to the TX1 and NPL-75 scales. Also presented are the results from direct comparisons of 3He and 4He realizations between 1,25 K and 3,2 K; these show the non-uniqueness of the ITS-90 in this region to be less than 0,3 mK.

Effects of wafer emissivity on lightpipe radiometry in RTP tools

We investigated the effect of different silicon wafer emissivities and the effect of low emissivity films on rapid thermal processing (RTP) wafer temperature measurements using lightpipe radiation thermometers (LPRTs). These tests were performed in the NIST RTP test bed. We used a NIST thin-film. thermocouple (TFTC) calibration wafer to calibrate the LPRTs in situ. The measurement; of LPRTs viewing Au and Pt thin-film spots in the center of the wafer were compared to LPRT radiance temperature readings that viewed bare Si/SiO/sub 2/. We found differences of up to 36/spl deg/C at 900/spl deg/C in the LPRT measurements due to the low emissivity films. A model of the wafer temperature measurement is presented to provide an insight into the effects of wafer emissivity on LPRT measurements in RTP tools.

ITS-90 calibration of radiation thermometers for RTP using wire/thin-film thermocouples on a wafer

Light-pipe radiation thermometers (LPRTs) are the sensor system of choice in RTP tools. They can be calibrated against blackbodies with an uncertainty (k=1) less than 0.3 °C. In an RTP tool, however, account must be made for wafer emissivity and wafer-chamber interreflections, or else temperature measurement uncertainties will be orders of magnitude higher. We have used two complementary approaches for accomplishing this: 1) in situ calibration using high-accuracy wire/thin-film thermocouples calibrated on the International Temperature Scale of 1990 (ITS-90) and 2) developing optical models to estimate the effective emissivity of the wafer eeff when used in the radiation environment of the RTP tool. The temperature measurement uncertainty of LPRTs using either technique is 2.1 °C or less.

Determining the thermal response time of temperature sensors embedded in semiconductor wafers

We present a non-contact method for the determination of the thermal response time of temperature sensors embedded in wafers. In this method, a flash lamp illuminates a spot on the wafer in periodic pulses; the spot is on the opposite side from the sensor under test. The thermal time constant of the sensor is then obtained from measurement of its temporal response, together with a theoretical model of heat flows both into the sensor and laterally within the wafer. Experimental data on both platinum resistance thermometers (PRTs) and on thermocouples embedded in silicon wafers show good agreement with the heat transfer models. Values of the thermal response time for a wide range of experimental parameters agree to within standard deviations of 8% (PRTs) and 20% (thermocouples), demonstrating the self-consistency of our results. The method is directly applicable to determining the thermal properties of sensors used in instrumented silicon wafers. We anticipate that the method will have use in development of new sensor attachment methods, in verifying the proper attachment of sensors during production, and in confirming that the thermal attachment has not degraded with age or thermal cycling. To simplify the application of the method, we have produced a table of calculated relevant quantities to be used in relating the measured signal to the thermal response time.

Normal and anomalous self-heating in capsule-type resistance thermometers in the range 1 K to 273 K

The normal self-heating coefficient for standard capsule-type resistance thermometers is gas-mediated over most of their designed range of use. This is readily modeled and calculated for thermometers with helium fill gas from known transport properties for temperatures above the superfluid-film transition at 1.3 K. Anomalous self-heating exists in certain capsules where air contamination has occurred, and this observed self-heating is compared with predictions from the thermal conductivity of gas mixtures. The experimental aspects of the self-heating measurements are described as well as certain aspects of the thermal transport modeling. Some associated interpolation errors are described and useful screening techniques are suggested for capsule thermometers over cryogenic temperature ranges.

Recent Advances in the Realization and Dissemination of the ITS-90 Below 83.8058 K at NIST

Recent advances in our knowledge of the International Temperature Scale of 1990 (ITS-90), as realized and maintained at the National Institute of Standards and Technology (NIST), are briefly reviewed. As a result of these advances, the NIST disseminated version of ITS-90 has recently undergone small adjustments below 83.8058 K. These adjustments are all at the sub-millikelvin level and reflect the inclusion of recent data on ITS-90 realizations and intercomparisons of reference thermometers. Specifically, developments in the realization of ITS-90 fixed points, gas thermometry, and vapor pressure thermometry at NIST have significantly improved our knowledge of scale non-uniqueness and dissemination uncertainty in the lowest temperature ranges. We briefly describe the present status of ITS-90 realizations at NIST and which definitions are currently being disseminated. Our procedures for calibration of standard reference thermometers for cryogenic use are reviewed and our updated assessments of calibration uncertainties are presented. These include calibration of Standard Platinum Resistance Thermometers (SPRTs) of the capsule type between 13.8033 K and 273.16 K, and of Rhodium-Iron Resistance Thermometers (RIRTs) between 0.65 K and 83.8058 K. In addition, we preview a new Standard Reference Material® (SRM®) project which will make available “as defined” ITS-90 calibrated capsule SPRTs through the MST SRM program. These continuing advances in scale realization and dissemination at NIST will improve the accuracy and availability of ITS-90 standards throughout the cryogenic engineering community.

BILATERAL KEY COMPARISON SIM.T-K6.1 ON HUMIDITY STANDARDS IN THE DEW/FROST-POINT TEMPERATURE RANGE FROM -25 °C TO +20 °C.

A Regional Metrology Organization (RMO) Key Comparison of dew/frost point temperatures was carried out by the National Institute of Standards and Technology (NIST, USA) and the National Research Council (NRC, Canada) between December 2014 and April, 2015. The results of this comparison are reported here, along with descriptions of the humidity laboratory standards for NIST and NRC and the uncertainty budget for these standards. This report also describes the protocol for the comparison and presents the data acquired. The results are analyzed, determining degree of equivalence between the dew/frost-point standards of NIST and NRC.